1. Field of the Invention
This invention relates to a constant-speed cruising control system for a vehicle, which makes the vehicle cruise at a desired constant speed.
2. Description of the Prior Art
Constant-speed cruising control systems for making vehicles cruise at a desired constant speed are known in the art. For example, in the system disclosed in Japanese Unexamined Patent Publication No. 58(1983)-192114, the vehicle is made to cruise at a desired constant speed by controlling the throttle opening according to the difference between the actual vehicle speed and the target vehicle speed (the desired speed). When the actual vehicle speed becomes lower than the desired vehicle speed by a predetermined value with the throttle valve fully opened, the automatic transmission is caused to downshift.
This system is disadvantageous in the following way. When the vehicle ascends a slope under the control of the system and the load on the engine increases, the system gradually increases the throttle opening to compensate for the increase in the engine load and causes the transmission to downshift when the throttle opening is maximized. When the actual vehicle speed reaches the target vehicle speed by virtue of the downshift, the system causes the transmission to upshift. Since the throttle opening is almost full at the time the upshift is effected, the upshift leads to a shortage in the engine output torque and a reduction in the vehicle speed. Then the system again causes the transmission to downshift. Thus the vehicle speed fluctuates up and down and hunting of the transmission occurs.
This problem may probably be overcome by taking into account the torque margins in controlling the throttle opening and the transmission. The torque margins can be determined by setting a threshold for the throttle opening. However, if the threshold, or preset value, is determined on the basis of the running resistance during running on an even road, which resistance is relatively small, hunting of the transmission occurs when the running resistance increases as when the vehicle ascends a slope and it becomes hard to converge the actual vehicle speed on the target vehicle speed. On the other hand, if the preset value is determined on the basis of a high running resistance, the transmission is caused to downshift even when the running resistance slightly increases while the vehicle runs on an even road, and fuel economy is deteriorated.
In view of these observations, we have proposed, in our U.S. Pat. application Ser. No. 32,236 now U.S. Pat. No. 4,833,612, to determine the preset value taking into account the conditions under which the vehicle runs, i.e. the running conditions, such as the inclination of the road on which the vehicle is running, road surface resistance or the like. However, this system has a drawback in that means for detecting the running condition is additionally required, which adds to the manufacturing cost of the system.